Monitoring of treatment responses and clonal evolution of tumor cells by circulating tumor DNA of heterogeneous mutant EGFR genes in lung cancer

• EGFR ctDNA rapidly responded to an EGFR-TKI as a response predictor.
• Transient ctDNA peaks of various EGFR mutants appeared during EGFR-TKI treatments.
• A pretreatment EGFR-mutant tumor harbors multiple activating and T790M mutations.
• T790M mutation does not necessarily confer resistance phenotypes to EGFR-TKIs.

ObjectivesEpidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) have dramatic effects on EGFR-mutant non-small-cell lung cancer (NSCLC). However, most patients experience disease recurrences, approximately half of which are T790M-mediated. Monitoring EGFR status with re-biopsy has spatiotemporal limitations.Patients and methodsEGFR circulating tumor DNA (ctDNA) in serial plasma samples was amplified and 105 of them were sequenced with a next-generation sequencer. Plasma mutation (PM) score was defined as the number of reads containing deletions/substitutions in 105EGFR cell free DNA (cfDNA).ResultsPM scores of various EGFR mutations showed dynamic, case-specific changes during EGFR-TKI treatments in 52 patients. The effects of the treatment on EGFR ctDNA were evaluated in 38 patients with elevated pre-treatment PM scores. The ctDNA responses correlated well with radiologic responses in radiologic good responders, whereas correlation was poor in non-responders. In addition to the peaks for the most prevalent ctDNA, small peaks of ctDNA with different types of activating EGFR mutations or the T790M mutation (early T790M ctDNA) appeared transiently in 10.5% and 26.3%, respectively. Early T790M ctDNA disappeared in all patients, including 7 who eventually developed acquired resistance accompanied by elevated levels of T790M ctDNA.ConclusionsMonitoring ctDNA is useful in evaluating treatment responses and monitoring driver oncogene status in NSCLC. ctDNA revealed clonal heterogeneity and genetic processes of cancer evolution in individual patients. The simple presence of the T790M mutation may be insufficient to confer EGFR-TKI resistance to tumor cells.